Frequency modulated wave detector



Oct. 21', 1941. s. HUNT 2,259,891

FREQUENCY MODULATED WAVE DETECTOR Filed Feb. 26, 1941 BY 7%ZM.

" ATTORNEY 9 .1. I 25 @l'? 4 v 74 30 I INVENTOR Patented Oct. 21, 1941 2,259,891 FREQUENCY MODULATED WAVE DETECTOR Seymour Hunt, Jackson Heights, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 26, 1941, Serial No. 380,649 5 Claims. (Cl. 2504:?)

My present invention relates to detectors of variable frequency waves, and more particularly to detectors of frequency, or phase, modulated carrier waves.

One of the main objects of my present invention is to provide a frequency modulated carrier wave detector which includes a rectifier having a pair of carrier energy input circuits; the input circuits being individually tuned to the carrier frequency and being adapted to develop carrier voltage in phase quadrature relation.

Another important object of the invention is to provide a driver tube for a frequency modulation detector, and one of the carrier-tuned input circuits of the detector being disposed in the cathode circuit of the driver tube.

Another object of the invention is to provide a frequency modulation detector which comprises a single diode having a load resistor in circuit with a pair of tuned circuits reactively coupled to provide phase quadrature relation between the voltages developed thereacross, and one of the tuned circuits being located in the cathode circuit of the preceding limiter tube.

Still other objects of my invention are to improve generally the simplicity and efficiency of frequency modulation detectors, and more especially to provide a detector of such type which is not only reliable, but is economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both'its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit diagram of one embodiment of the invention,

Fig. 2 vectorially shows the functioning of the discriminator section of the detector network,

Fig. 3 illustrates a modification; Fig. 4 is another form.

Referring now to the accompanying drawing, wherein like reference characters in the figures designate similar circuit elements, the numeral I indicates a pentode, say of the GE? type, acting as an intermediate frequency amplifier. For this purpose the input transformer 2 has its primary and secondary windings each resonated to Fcthe centerfrequency of the frequency modulated carrier waves. Similarly, the output transformer 3 has its primary and secondary winding each tuned to Fe. The receiver being of the superheterodyne type and having its signal selector circuits tunable over the assigned frequency range of 42 to 50 megacycles (me), there will be used an I. F. value chosen from a range of 2 to, 8 me. For example, a value of 4.3 mc. may be used for the I. F. magnitude. Hence, Fe will be 43 mo.

The usual tunable radio frequency amplifier may precede the converter stage whose output circuit is adjusted to deliver I. F. energy to transformer 2 of the desired Fc value. As is well known, the audio modulation appears in the frequency modulated carrier Wave as a carrier frequency deviation. The audio amplitude variation corresponds to carrier frequency deviation from Fe, while the audio frequencies per se correspond to the rate of deviation from Fe. Since a purely frequency-variable carrier wave must be delivered to the detector input, or discriminator, network, a carrier amplitude variation limiter is used prior to the detector. Noise impulses, prior resonant circuits and fading will each cause amplitude variation efiects to appear in the frequency-variable carrier. The limiter acts to eliminate the amplitude variation so as to insure delivery of a constant amplitude-frequency variable carrier to the discriminator.

The tube 4, which may be of the 6R7 type, has a cathode 5, a control grid 6 and a plate 1. Adjacent cathode 5 is arranged an auxiliary anode 8. The grid 6 is connected to the high potential side of the input circuit, while resistor 9, shunted by I. F. bypass condenser I 0, is connected between the low potential side of the input circuit and ground. The plate 1 is connected to a source of positive direct current potential through resistor I Leach end of the latter being bypassed to ground by a capacity.

The cathode 5 is connected to ground through coil l2 of tuned circuit l2l3 resonated to Fe- The anode 8 is connected to ground through a series path comprising coil 14 and load resistor I6. The coil I 4 is shunted by condenser l5, and is tuned to the value of Fe. Coils l2 and I4 are magnetically. coupled. Load resistor I6 is bypassed for I. F. currents by condenser I6. The audio, or modulation, voltage developed across resistor I6 is taken off through audio bypass condenser l1.

Considering, first, the limiter section of tube 4, it is pointed out that rectification takes place in the grid circuit of tube 4, and a direct current voltage is built up across the network 9-40. This voltage biases the grid 6, and tends to limit the plate current flow. Resistor l l in series with the B supply to the plate I also tends to limit the plate current. In this way there is secured a limitation of carrier amplitude variation. The direct current voltage across 9 may also be used for the usual AVC function. The plate current flows through the coil I2 of the tuned primary circuit l2-l3, and develops across circuit 12-43 a radio frequency voltage which is vectorially represented by the symbol E1 in Fig. 2. A second voltage E2 is induced in the secondary tuned circuit l4l5 by virtue of the magnetic coupling between coils l2 and I4. In Fig. 2 the voltage E2 is shown as 90 degrees out of phase with the voltage E1. This is the phase relation which exists at the frequency value Fe. Both voltages E1 and E2 are rectified by diode 8-5, and the diode current which flows through load resistor I6 is proportional to the vector sum of voltages E1 and E2. In Fig. 2 this vector sum is represented by the dotted arrow E3.

As the center frequency Fe of the frequency modulated carrier waves shifts its frequency in relation to the applied modulation the angle between E1 and E2 shifts. As the angle shifts the length of vector E3 increases and decreases. That is, the coupled circuits I2-I3 and [4-45 function as a discriminator which converts the frequency-variable carrier into a corresponding carrier of constant frequency but of variable amplitude. Furthermore, the variable amplitude corresponds to the audio modulation applied to the carrier.

In Fig. 3 there is shown the arrangement of Fig. 1 in a balanced rectifier circuit. Tube 4 has two auxiliary anodes 8 and 8 adjacent cathode 5. The midpoint of coil M is grounded. The load resistor l6 has a grounded center tap. The anodesB and 8 are connected to opposite sides of tuned circuit l4l5 through coupling condensers. Radio frequency chokes are in series with the coupling condensers. The action is similar to that of Fig. 1, except that balanced rectification is secured.

In Fig. 4 there is shown a modification of the arrangement shown in Fig. 1 wherein the rectifier associated with the discriminator is of the balanced type. The midpoint of coil I4 is connected through coil 23 to the junction of resistors SI and 24, whereas the opposite ends of coil M are connected to the anodes respectively of diodes 3D and 2B. The load resistor 3| is con nected between the anode and cathode of diode 3i and it will be noted that the load resistors 2! and 24 are arranged in series relation. The cathode end of resistor 31 is connected to an audio frequency utilization network through a condenser 55 which has a low impedance to audio frequency voltage. The cathode end of resistor 24 is grounded, and each of load resistors 3i and 24 is by-passed for I. F. The coils l2 and M are magnetically coupled as in Fig. 1. In this arrangement a special bias resistor 4|, shunted by an I. F. by-pass condenser 42, is arranged in series between the cathode of tube 4 and coil l2. The direct current blocking condenser 4 l is connected between the high side of coil I2 and midpoint of coil M. The operation in this arrangement is similar to that described above. Across circuit l4l5 is developed carrier voltage which is in phase quadrature at F with the voltage across I2--l3. The voltages across resistors 3| and 24 are in polarity opposition so that at F0 zero voltage is developed across these load resisters.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In an angular velocity-modulated carrier wave detection system, a tube provided with in put and output electrodes, a modulated carrier voltage input circuit connected to the input electrodes, an output circuit tuned to the wave center frequency and common to the said input and output electrodes, a second circuit tuned to said center frequency and reactively coupled to said tuned output circuit whereby carrier voltages across the coupled circuits are in phase quadrature at the center frequency, means for rectifying the carrier voltage across each of the coupled tuned circuits, and means for combining the rectified voltages in polarity opposition. I

2. In an angular velocity-modulated carrier wave detection system, a tube provided with input and output electrodes, a modulated carrier voltage input circuit connected to the input electrodes, an output circuit tuned to the wave center frequency and common to the said input and output electrodes, 9. second circuit tuned to said center frequency and reactively coupled to said tuned output circuit whereby carrier voltages across the coupled circuits are in phase quadrature at the center frequency, a single diode means for rectifying the carrier voltage across each of the coupled tuned circuits, and means for combining the rectified voltages in pdlarity opposition.

3. In an angular velocity-modulated carrier wave detection system, a tube provided with input and output electrodes, a modulated carrier voltage input circuit connected to the input electrodes, an output circuit tuned to the wave center frequency and common to the said input and output electrodes, a second circuit tuned to said center frequency and reactively coupled to said tuned output circuit whereby carrier voltages across the coupled circuits are in phase quadrature at the center frequency, a single diode means for rectifying the carrier voltage across each of the coupled tuned circuits, and means for combining the rectified voltages in polarity opposition, said last means consisting of a single resistor in series with the coupled tuned circuits.

4. In a frequency modulated carrier wave detection system, a tube provided with input and output electrodes, a modulated carrier voltage input circuit connected to the input electrodes, an output circuit tuned to the carrier wave center frequency and common to the said input and output electrodes, a second circuit tuned to said center frequency and reactively coupled to said tuned output circuit whereby carrier voltages across the coupled circuits are in phase quadrature at the center frequency, means including diode anodes in said tube for rectifying the carrier voltage across each of the coupled tuned circuits, and means for combining the rectified voltages in polarity opposition to derive the modulation voltage.

5. In a phase, or frequency, modulated carrier wave detection system, a tube provided with input and output electrodes, a modulated carrier voltage input circuit tuned to the carrier wave center frequency connected to the input electrodes, an output circuit tuned to the center frequency and common to the said input and out put electrodes, a second circuit tuned to said center frequency and reactively coupled to said tuned output circuit whereby carrier voltages across the coupled circuits are in phase quadrature at the center frequency, a pair of opposed rectifiers connected across the second tuned circuit for rectifying the carrier voltage across the second tuned circuit, and means for combining the rectified voltages in polarity opposition thereby to provide the modulation voltage.

SEYMOUR HUNT. 

